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An SDN overlay is a deployment method for network virtualization and software-defined networking (SDN) that involves running a logically separate network or network component on top of existing infrastructure.
Big Switch Networks’ Big Virtual Switch is one example of an SDN overlay application. Big Virtual Switch makes it possible to run a software-defined network on top of any infrastructure, whether or not it’s compatible with OpenFlow, the de facto standard protocol for SDN controllers and switches.
Specifically, Big Switch’s network overlay technology uses OpenFlow-enabled hypervisor virtual switches at the server access layer of a network to create a virtual network on top of an existing physical network. The network overlay tunnels through the physical network. That makes it possible to build a software-defined network on top of infrastructure that does not explicitly support SDN. Big Switch’s technology is also designed to work with the various tunneling protocols used by other vendors.
Other methods of deploying software-defined networking include pure OpenFlow networks and hybrid SDN networks.